The present invention relates to a flat blank from which to form a rigid packet.
The flat blank in question may be used to advantage, though not exclusively, for producing a rigid cigarette packet, and normally comprises a cup-shaped bottom portion or container and a top lid connected to each other by a hinge.
Known rigid packets of the type described above normally have a rectangular section, though rigid cigarette packets have also been proposed with other, e.g. octagonal or similar, sections.
A rigid packet of the type described above is known to be produced from a flat, substantially rectangular cardboard blank having a number of longitudinal and transverse fold segments, along which the blank is folded to form the packet, in which the longitudinal fold segments normally define, laterally, two parallel major lateral walls and two parallel minor lateral walls perpendicular to the major walls.
Often, the walls of the finished packet, in particular the major walls, are not perfectly flat, on account of the tendency of the blank, when folded along a fold segment, to spring back to its original flat shape.
To reduce springback, it has been proposed to equip packing machines with weakening units for prefolding and weakening the blanks along the fold segments. Though effective, such a solution has been found to involve considerable cost, on account of the complex mechanical design of the weakening units.
It is an object of the present invention to provide a flat blank enabling straightforward, low-cost production of a rigid packet designed to eliminate the aforementioned drawback, i.e. a packet with substantially flat walls.
According to the present invention, there is provided a flat blank from which to produce a rigid packet, the blank comprising a number of preweakened fold segments, and being characterized by said fold segments having at least two different degrees of weakness.
Said fold segments are preferably so located as to form respective sharp edges once the blank is folded to form said packet.
Tests have surprisingly shown that, regardless of the shape of the packet, curving of the walls of the packet due to springback of the blank is substantially eliminated by weakening some fold segments more than others. Which fold segments are to be weakened most to achieve the desired effect obviously depends on the shape of the packet, and must be established substantially by trial and error. Nevertheless, tests have shown that any type of packet has a particular combination of fold segments which, if weakened to a greater degree, provide for obtaining almost perfectly flat walls without impairing the shape stability of the packet.
Take, for example, the case of a substantially octagonal-section packet formed from a substantially rectangular blank comprising longitudinal and transverse fold segments, and wherein the longitudinal fold segments are aligned along at least two inner longitudinal and two outer longitudinal fold lines. Such a packet is defined by a cup-shaped body and by a lid hinged to the cup-shaped body; the cup-shaped body and the lid have respective major lateral walls and respective minor lateral walls; and the transverse fold segments divide a portion of the blank extending between the two inner longitudinal fold lines into a first panel corresponding to a first major lateral wall of the cup-shaped body, and a second panel corresponding to a second major lateral wall of the cup-shaped body.
In this particular case, the walls of the packet can be flattened by weakening to a greater degree than the other fold segments of the blank the two longitudinal fold segments located on either side of the first panel and each defining a portion of a respective inner longitudinal fold line, and the two longitudinal fold segments located on either side of the second panel and each defining a portion of a respective outer longitudinal fold line.